At a campaign rally in Chicago for Alexi Giannoulias, the Democratic nominee for U.S. Senate in Illinois, Obama referred to media reports that foreign contributions are being used by conservative groups in the Illinois Senate campaign.
"So the question for the people of Illinois is, are you going to let special interests from Wall Street and Washington and maybe places beyond our shores come to this state and tell us who our senator should be?," Obama said.
"That's not just a threat to Democrats. That's a threat to our democracy," he said. "The American people deserve to know who's trying to sway their election. And if we just stand by and allow special interests to silence anybody who's got the guts to stand up against them, our country is going to be a very different place."
Giannoulias is running for the Senate seat Obama vacated after he was elected president in 2008.
Obama appeared earlier Thursday at Bowie State University in Bowie, Md., with Gov. Martin O'Malley, who is seeking re-election. The New York Times said the rally drew several thousand college students and local residents.
"I'm going to need you just as fired up as you were in 2008," Obama told the crowd.
In a scene reminiscent of his State of the Union address, a heckler yelled "You're a liar!" at the president, the Times said. But people nearby shouted the heckler down, making it difficult for many in the audience to hear what Obama was saying for several minutes, the report said.
O'Malley is facing the man he defeated in 2006, former Gov. Robert L. Ehrlich Jr.